Herpes simplex virus type 1 was inactivated by illumination with red light in the presence of low concentrations (1 microM) of methylene blue (MB). In ultrastructural examinations after photodynamic treatment, no significant damage of the viral envelope was observed. The inactivated viruses were able to bind to host cells and to penetrate them. Viral DNA was released within the cells, but DNA replication was completely inhibited by photodynamic treatment. In Southern blots, a degradation of the viral DNA was detected with increasing illumination time. Taken together, these data show that MB/light treatment of herpes viruses gives rise to DNA damage and blocks DNA replication.